Two reasons to leave them open
The changes in Dynamo cause old solutions to fail and sometimes require small tweaks like in this recent post that revived a two year old topic. Wouldn’t make sense creating a new topic for something like this.
Practical implementation of solutions on site should be encouraged. Closing posts would prevent such posts, a year or more later
However, moderators could be encouraged to proactively close topics that deserve closure in their opinion. Just my opinion
I must also confess that it feels nice when some really old post is revived. Good to know that your ancient contributions are still helping someone 